A diversity of linearly reciprocating mechanisms and so on installed in recent years in semiconductor manufacturing equipment, precision machines, measurement/inspection instruments, medical equipment, robots, assembling machines, transporting machines, micromachines and so on is designed compact or reduced in construction in favor of energy saving or high energy efficiency. Correspondingly, linear motion guide units built in relatively sliding parts used in the machinery stated just earlier are also needed compact or downsized in construction while expected to carry heavy loads and make sure of smooth movement of the slider relative to the guide rail. The roller-type linear motion guide units are advantageous to heavier load-carrying capacity, but they are limited in downsizing or miniaturization of the slider. One of major challenges in the roller-type linear motion guide unit, thus, resides in downsizing in construction of the slider, especially making the slider short and small in size.
In a commonly-owned Japanese Laid-Open Patent Application No. H09-72 335 there is disclosed a linear motion guide unit in which a circular sleeve to provide a return passage fits into a lengthwise bore made in a carriage of a slider. The sleeve is made to undergo elastic deformation. More especially, the sleeve is anchored at lengthwise ends thereof in the bore in the carriage and reduced radially at the middle between the ends to make a circular clearance to isolate a circular surface of the sleeve away from the circular inside of the bore. The sleeve is further made with a lengthwise slit to undergo easily elastic deformation at the middle of the sleeve. Thus, the sleeve constructed as stated just earlier helps absorb or alleviate sliding resistance applied on the rolling elements to make sure of smooth running or movement of the rolling elements.
With the prior linear motion guide unit constructed as stated earlier, the sleeve for the return passage is great in the overall length and, correspondingly, the radially reduced middle thereof susceptible to the elastic deformation is also longer relatively to the radially thick ends. With the downsized roller-type linear motion guide unit, however, the carriage built in the slider needs to be downsized or compact in construction, especially short in length. As it does, the sleeve inserted into the bore in the carriage has to be made miniature and short in length as well. That in turn will show that the sleeve at the radially reduced middle thereof becomes too less in thickness to ensure the durability of the sleeve. Where making the sleeve at the radially reduced middle thereof greater in thickness in favor of the durability of the sleeve, the sleeve short in length is hard to undergo the elastic deformation at the reduced middle. As a result, it remains a major challenge to the prior downsized linear motion guide unit to make certain of smooth relative sliding movement of the slider.